In conjunction with low and middle altitude aerial photography, as applied to either single vertical exposures or stereo photogrammetric ground mapping, pre-marking of the ground to facilitate the photo mission and the interpretation of the photos subsequently is a commonly accepted practice. The ground control marks which are provided in the ground marking serve to assist the pilot in a more precise identification of the positions of control points on the ground, as viewed from the air, as well as assisting in recognizing these same points in the photograph for subsequent analysis. Ground pre-marking has been a long accepted practice in ground point identification.
Aerial panels used today for ground marking typically are shaped in a cross, a xe2x80x9cVEExe2x80x9d, a xe2x80x9cTEExe2x80x9d, or a xe2x80x9cWYExe2x80x9d. By far the most widely used configuration is a symmetrical cross, which in most cases is the easiest to distinguish amidst the various shapes and hues of the typical, vertical black-and-white ground photography.
The sizes of the ground marking panels are dependent upon flight altitude, which in turn depends on the mapping scale. Photogrammetrists differ in their preferences for the sizes of targets. The effective size of the ground marking panel also is largely dependent on anticipated shading conditions on the ground, namely contrast, being the most important element in visual point acquisition. Clearly, the sizes are proportional to the altitude in most cases.
At altitudes of less than 1,000 feet above ground level (AGL), panel sizes (xe2x80x9csectionxe2x80x9d or arm-length vs. width) may be as small as 12xe2x80x3xc3x973xe2x80x3, particularly when the panel is painted. Usually, white is used for black asphalt surfaces, or when a cross is applied to black cloth or felt paper. At the highest mapping altitudes, panel sections may reach 20xe2x80x2xc3x972xe2x80x2, or larger. At ultra-high altitudes, such as those used for aerial mosaic work, pre-marking is not possible. Control point identification in such high altitude situations normally is accomplished by identifying objects which clearly appear in the photography after the photo mission, such as street intersections, a building, or the corner of a cultivated field. This latter process, effected by identifying objects on the photo to provide marking points, is known as xe2x80x9cphoto identificationxe2x80x9d; and it also frequently is applied to high and middle altitude projects where the precision requirements have been somewhat relaxed.
Where a more precise ground point identification is required, however, accurate placement and identification of the ground marking panels is necessary. Except in cases where the surveyor or the engineer has latitude to vary the size of the panel, such as paint on pavement, the ground marking practice usually depends upon the availability of commercially produced aerial flagging made of white polyethylene plastic. This plastic material typically is available in 6xe2x80x3, 8xe2x80x3, 12xe2x80x3 and 18xe2x80x3 widths. It generally is provided in rolls of these widths, which are cut to length by the surveyor or engineer at the ground marking point. Also in common use, is pre-manufactured, re-usable cloth panels. These typically have a 3xe2x80x3 wide white cross or iron cross on a 4xe2x80x2xc3x974xe2x80x2 black background.
A common denominator of all pre-marking applications, regardless of the material used, or its size or shape, is that the panel must be securely fastened to the ground. The ground, with all of its irregularities, is the most notable obstacle in panel placement. The two traditional methods of placing existing plastic and cloth materials are either by burying or weighting the corners or ends, or by direct nailing. Nailing directly through a cloth panel along the edges of the tightly stretched cloth material usually is sufficient for the job.
Rolled plastic material, however, cannot be nailed directly through the material, without a backing, due to the brittle nature of the plastic and its succeptability to tearing. A common practice in the use of plastic material is to roll both ends of each section of the panel with a shortened lath stake (typically, a two-man operation). The material is rolled two or three times around the stake. Then, each end of the tightly stretched section is fastened to the ground with nails driven through the rolled stake. The panel is finished by repeating the process, with a second section crossing the first. Generally, nails used in this type of process are whatever is available in the survey crew truck, and NO. 16 and NO. 20 box nails generally are the sizes used.
In addition to the foregoing difficulties, centering the panel over an existing control point or surveyor monument, under normal circumstances, is a matter of judgement of the installer. The installer simply estimates, or actually measures to center the panel over the point from both directions. When the monument projects above the surrounding ground level, a crude hole is cut in the material, allowing for the protrusion of the monument through the hole. This usually sacrifices the integrity of the panel material around its center, through which the monument projects.
Polyethylene plastic sheets and cloth also are very vulnerable to the wind. Consequently, it is extremely important to securely fasten the panel to the ground. In a typical situation, the time between the placement of the surveyor""s panel and the photogrammetrists photo mission may be several days, and perhaps even several weeks. It is imperative that the panel is set in such a way as to be taut, as nearly flush as possible to the ground, and fastened tightly. The combination effects of the sun expanding the plastic and the wind""s buffeting (particularly of the top section) can destroy the positional accuracy of the panel and/or the photo-visibility of the panel in a relatively short time.
Another problem which exists with all types of aerial photo marking, regarding visibility, is that the effectiveness of the panel is significantly dependent upon the plane of the immediate terrain in which the control point lies. An ideal situation locates the panel at right angles to the photography, or as nearly as possible to horizontal. With vertical exposures, this assumes that the photo mission will take place at such time when the sun is highest in the sky. This results in the maximum illumination of the target (panel).
The terrain on which the panel is placed is almost never truly level, since many of the targets on any given project are situated on hillsides or near ground cover interference. As a result, the contrast of the panel with the surrounding land is frequently quite low. This is commonly a result of sunlight being deflected from the panel. Conversely, on projects with lighter shaded backgrounds, such as sand, decomposed granite, or concrete, illumination of the panel is not desirable; and deflected or absorbed light (such as effected by black paint) is necessary for the highest degree of contrast. Since the panel usually occupies a single plane, the entire panel may be all but lost in typical imagery of non-uniform ground conditions.
The United States patent to Ladd U.S. Pat. No. 1,507,191 is directed to a marker for assisting aviators in flying cross country. This is not a surveyor ground pre-marking device. The marker disclosed in this patent, however, is expensive and complex. It requires the erection of a rather substantial tower, on the top of which is provided indicia to be used by aviators.
Another patent directed to providing visibility from the air for a particular location is Barnes U.S. Pat. No. 1,796,233. This patent is directed to a building roof which has transparent/reflective and opaque sections on it to facilitate its visibility from the air. This is not a surveyor panel or surveyor pre-marker in any sense.
Other patents have been developed for marking a location for identification from the air utilizing complex optical and electronic devices. Some of these devices are used for aerial survey systems or in aerial viewing systems. United States patents directed to such systems are McConnell U.S. Pat. No. 3,766,312; Masterfield U.S. Pat. No. 4,041,529; and LaPeyre U.S. Pat. No. 4,497,551. These are not portable, disposable markers of the type provided by the polyethylene rolled material, or the cloth material described above.
A number of patents are directed to what are known as xe2x80x9cdrop marker systemsxe2x80x9d for marking locations on the ground from an aircraft. These markers are dropped from the aircraft; so that on a subsequent pass over that same area, the aircraft can see the marker identifying the location of the drop. Such drop markers are used in crop dusting operations and the like to mark the end of each pass. The pilot then has a particular visual reference to the location of the previous pass, when a new pass is being made. A number of patents directed to drop markers, which have fan-fold panels for visibility by a pilot who is crop dusting, utilize devices including a base member and some type of flexible corrugated board or ribbon attached to it for release and deployment when the base member is dropped from the airplane. United States patents directed to such systems are Tillay U.S. Pat. No. 3,428,019; Huwe U.S. Pat. No. 3,973,513; Taylor U.S. Pat. No. 4,261,283; Forsyth U.S. Pat. No. 4,270,482; Huwe U.S. Pat. No. 4,300,469; Lloyd U.S. Pat. No. 4,316,554; and Loeckle U.S. Pat. No. 4,474,349. None of the devices disclosed in these patents, however, is designed to be applied by persons on the ground. They simply amount to packages or markers dropped from an airplane, which then are designed to elongate in the airstream as they fall from the plane. Thus, upon landing, they are visible from the aircraft. Obviously, precise location of these devices at a surveyed marker point on the ground is not intended.
It is desirable to provide an improved high-visibility aerial ground marking panel, which is easily applied at a precise marking point, which is compact and inexpensive, and which provides enhanced visibility in light sources at various angles.
It is an object of this invention to provide a high visibility aerial panel system.
It is another object of this invention to provide an aerial panel pre-marking device which is simple and easy for one person to install.
It is an additional object of this invention to provide a ground marking panel providing high visibility at different light angles and which is easy to install.
It is a further object of this invention to provide a high visibility pre-marking panel for use with aerial mapping which is compact, which can be installed accurately and easily, and which provides enhanced visibility from the air under various lighting conditions.
In accordance with a preferred embodiment of the invention, a high visibility ground marking panel, for use in photogrammetric ground mapping, is made of a main body of relatively thin, lightweight corrugated cardboard, the length of which is substantially greater than its width. A hole is formed at the midpoint (both longitudinally and transversely) of the panel body; and the panel body is folded alternately into an accordion type fold for compact storage and subsequent installation. More specific embodiments of the invention include longitudinal and transverse perforations to enable the panel to be split into smaller sections when the full length and full width of the panel are not required for any particular job application. In installation of the panel, the ends are drawn outwardly from the center marker point and are secured to the ground; so that the alternately folded sections diverge at predetermined angles from the ground plane. This enhances the visibility of the panel from the air, and from various angles and under different light conditions.